# Apparent ecosystem carbon turnover time: uncertainties and robust features

The turnover time of terrestrial ecosystem carbon is an emergent ecosystem property that quantifies the strength of land surface on the global carbon cycle–climate feedback. However, observation- and modeling-based estimates of carbon turnover and its response to climate are still characterized by large uncertainties. In this study, by assessing the apparent whole ecosystem carbon turnover times (inline-formulaτ) as the ratio between carbon stocks and fluxes, we provide an update of this ecosystem level diagnostic and its associated uncertainties in high spatial resolution (0.083inline-formula) using multiple, state-of-the-art, observation-based datasets of soil organic carbon stock (inline-formulaCsoil), vegetation biomass (inline-formulaCveg) and gross primary productivity (GPP). Using this new ensemble of data, we estimated the global median inline-formulaτ to be inline-formula $M6inlinescrollmathml{normal 43}_{-normal 7}^{+normal 7}$ 24pt17ptsvg-formulamathimga964b3b30c1860cbfb56e35448624327 essd-12-2517-2020-ie00001.svg24pt17ptessd-12-2517-2020-ie00001.png inline-formulayr (inline-formula $M8inlinescrollmathml{\text{median}}_{-\text{difference to percentile 25}}^{+\text{difference to percentile 75}}$ 135pt20ptsvg-formulamathimg8a54348845f175bc742186629122fb63 essd-12-2517-2020-ie00002.svg135pt20ptessd-12-2517-2020-ie00002.png ) when the full soil is considered, in contrast to limiting it to 1 inline-formulam depth. Only considering the top 1 inline-formulam of soil carbon in circumpolar regions (assuming maximum active layer depth is up to 1 inline-formulam) yields a global median inline-formulaτ of inline-formula $M13inlinescrollmathml{normal 37}_{-normal 6}^{+normal 3}$ 24pt17ptsvg-formulamathimg066305b928331c7b879dc48e187744a3 essd-12-2517-2020-ie00003.svg24pt17ptessd-12-2517-2020-ie00003.png inline-formulayr, which is longer than the previous estimates of inline-formula $M15inlinescrollmathml{normal 23}_{-normal 4}^{+normal 7}$ 24pt17ptsvg-formulamathimg9e1c1f4bd0e302dbebbbe024a3cf245f essd-12-2517-2020-ie00004.svg24pt17ptessd-12-2517-2020-ie00004.png inline-formulayr (Carvalhais et al., 2014). We show that the difference is mostly attributed to changes in global inline-formulaCsoil estimates. inline-formulaCsoil accounts for approximately 84 % of the total uncertainty in global inline-formulaτ estimates; GPP also contributes significantly (15 %), whereas inline-formulaCveg contributes only marginally (less than 1 %) to the total uncertainty. The high uncertainty in inline-formulaCsoil is reflected in the large range across state-of-the-art data products, in which full-depth inline-formulaCsoil spans between 3362 and 4792 inline-formulaPgC. The uncertainty is especially high in circumpolar regions with an uncertainty of 50 % and a low spatial correlation between the different datasets (inline-formula $M24inlinescrollmathmlnormal 0.2 63pt10ptsvg-formulamathimg0c74f3c5f44fa4a72ab00f103017d905 essd-12-2517-2020-ie00005.svg63pt10ptessd-12-2517-2020-ie00005.png ) when compared to other regions (inline-formula $M25inlinescrollmathmlnormal 0.6 63pt10ptsvg-formulamathimg0c48fe27e45d5730a498149c8387e90b essd-12-2517-2020-ie00006.svg63pt10ptessd-12-2517-2020-ie00006.png ). These uncertainties cast a shadow on current global estimates of inline-formulaτ in circumpolar regions, for which further geographical representativeness and clarification on variations in inline-formulaCsoil with soil depth are needed. Different GPP estimates contribute significantly to the uncertainties of inline-formulaτ mainly in semiarid and arid regions, whereas inline-formulaCveg causes the uncertainties of inline-formulaτ in the subtropics and tropics. In spite of the large uncertainties, our findings reveal that the latitudinal gradients of inline-formulaτ are consistent across different datasets and soil depths. The current results show a strong ensemble agreement on the negative correlation between inline-formulaτ and temperature along latitude that is stronger in temperate zones (30–60inline-formula N) than in the subtropical and tropical zones (30inline-formula S–30inline-formula N). Additionally, while the strength of the inline-formulaτ–precipitation correlation was dependent on the inline-formulaCsoil data source, the latitudinal gradients also agree among different ensemble members. Overall, and despite the large variation in inline-formulaτ, we identified robust features in the spatial patterns of inline-formulaτ that emerge beyond the differences stemming from the data-driven estimates of inline-formulaCsoil, inline-formulaCveg and GPP. These robust patterns, and associated uncertainties, can be used to infer inline-formulaτ–climate relationships and for constraining contemporaneous behavior ofpage2518 Earth system models (ESMs), which could contribute to uncertainty reductions in future projections of the carbon cycle–climate feedback. The dataset of inline-formulaτ is openly available at https://doi.org/10.17871/bgitau.201911https://doi.org/10.17871/bgitau.201911 (Fan et al., 2019).

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Fan, Naixin / Koirala, Sujan / Reichstein, Markus / et al: Apparent ecosystem carbon turnover time: uncertainties and robust features. 2020. Copernicus Publications.

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